For wireless communication, multiple-input and multiple-output, or MIMO, technology using a plurality of antennas has been proposed in recent years. According to this technology, a pilot signal is transmitted from each transmit antenna to secure propagation channels for the transmit antenna.
In this case, at an instant when one antenna is transmitting a pilot signal, other antennas are unable to perform transmission. Therefore, an increase in the number of antennas incurs increased pilot signal overhead other than data transmission at the cost of reduced transmission efficiency.
Note that there has been conventionally proposed a method for selecting antennas in a space-division multiple access (SDMA) wireless network that includes a base station and a set of mobile stations (see Japanese National Publication of International Patent Application No. 2010-527165, for example).
There has been also conventionally proposed a beam forming method for an improved two-dimensional smart antenna array suited for used in a cellular base station (see Japanese Laid-open Patent Publication No. 2003-60423, for example).
As described above, the MIMO wireless communication has a problem that the pilot signal overhead increases as the number of antennas increases, causing a decrease in data transmission efficiency.